The MPEG 1 Audio standard as specified in ISO/IEC 11172-3 defines three operational modes known as Layers I, II and III. Each Layer offers increased compression but also increased encoding complexity, whereby downward compatibility is guaranteed. Therefore, a Layer II decoder can also read a Layer I data stream but no Layer III data stream. Also a Layer III decoder can decode all MPEG 1 Audio bit streams, i.e. Layer I to III.
MPEG 1 Audio data compression is based on subband coding. The audio signal is split into 32 subbands of equal width. A quantization is performed using a psychoacoustic model that is adapted to the masking behaviour of the human hearing. Each subband signal is quantized in such a way that the quantization noise introduced by the coding will not exceed the masking curve for that subband. After quantization the samples build—together with the scale factors and further coding information—a frame structure for transmission.
In ISO/IEC 11172-3 two independent psychoacoustic models are defined which can be adapted to any Layer. The output from these psychoacoustic models is a set of Signal-to-Masking Ratios, SMRn, for every subband n.
In order to calculate the SMRn for the psychoacoustic model 2 a Fast Fourier Transformation (FFT) with a length of 1024 samples is used, which has to run two times per channel, i.e. four times for a stereo channel.